1. Field of the Invention
This invention relates to an apparatus and a method for purifying an exhaust gas in an internal combustion engine and in particular, for estimating an amount of poisoning substance deposited in an exhaust gas purification catalyst during controlling removal of the poisoning substance.
2. Related Art of the Invention
There is known an apparatus for estimating an amount of poisoning substance (sulfur) deposited in an exhaust gas purification catalyst to remove the poisoning substance that is disclosed in a Japanese Unexamined Patent Publication No. 2001-227333 and 2000-73744.
The Publication No. 2001-227333 discloses that removal control of the poisoning substance to make an air-fuel ratio to be in a rich range is carried out for a predetermined period and thereafter, the removal control ends.
However, a relation between a temperature of a catalyst and a poisoning substance regeneration rate is not factored in for the removal control and as a result, the amount of the poisoning substance is not accurately estimated. Therefore, the removal control ends regardless of incomplete removal or continues to be carried out regardless of complete removal, causing deterioration of fuel economy and exhaust gas emission performance.
The Publication No. 2000-73744 discloses that an amount of a poisoning substance is estimated by subtracting a removal amount of the poisoning substance calculated based upon an engine operating condition in a predetermined period after the removal control starts.
Therefore, the amount is finally estimated as 0 depending on the elapse time wherein it is not factored in that the removal amount of the poisoning substance has a limitation depending on a catalyst temperature and an air-fuel ratio and as a result, the amount of the poisoning substance is not estimated accurately.
Accordingly, ending the poisoning substance removal control is too early and too long, causing deterioration of fuel economy and exhaust gas emission performance.